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Generating Triangle Waves

In the basic square wave generator circuit, a gradually-charging capacitor was used to help set the timing or frequency of the circuit. However, since it was only charging through a resistor, it necessarily charged on a logarithmic curve, rather than as a linear ramp. Can we use an op amp integrator here to obtain a linear triangle wave along with the square wave?


A generator for both square and triangle waves

In the circuit to the right, we use a separate integrator to generate a ramp voltage from the generated square wave. As a result, we can get both waveforms from a single circuit. The phase relationship shown between the two output waveforms is correct — remember that the integrator inverts as well as integrating, so it will produce a negative-going ramp for a positive input voltage, and vice-versa.

Because we are now using an op amp integrator to get the triangle wave, we no longer have a logarithmic response anywhere in the circuit. Therefore, the equation for the operating frequency is simplified to:

fout 1  (  R2  )


4RtC R1

The squarewave amplitude is still the limit of voltage transistion, which we are assuming here to be ±10 volts. The triangle wave's amplitude is set by the ratio of R1/R2. Note that for this circuit to function, it is necessary that R1 be less than R2. This keeps the triangle amplitude less than the square wave amplitude. It is also necessary for the resistor values to be within a reasonable range for correct operation of the op amps. There is no restriction on the value of C.


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